An optical fiber typically can support two or more propagation modes for an optical signal. For example, the light energy comprising the optical signal can travel in either the LP.sub.01 or the LP.sub.11 spatial propagation mode of a two-mode optical fiber. As another example, the light energy of an optical signal can travel in either of two orthogonal polarization modes in a birefringent single-mode optical fiber. In either of the two examples presented, the optical fiber provides two propagation paths for the optical signal, and one of the propagation paths in each example is faster than the other propagation path (i.e., the LP.sub.11 propagation mode is faster than the LP.sub.01 propagation mode, and light polarized along one axis of a birefringent fiber travels faster than light polarized along the other axis of a birefringent fiber). These known phenomena have been used to advantage to couple light between the two propagation modes by periodically stressing the fiber. For example, see "Active Polarization Coupler for Birefringent Fiber," J. L. Brooks, et al., OPTICS LETTERS, Vol. 9, No. 6, June 1984, pp. 249- 251; "Birefringent Fiber Polarization Coupler," R. C. Youngquist, et al., OPTICS LETTERS, Vol. 8, No. 12, December, 1983, pp. 656-658; "Two-Mode Fiber Modal Coupler," R. C. Youngquist, et al., OPTICS LETTERS, Vol. 9, No. 5, May, 1984, pp. 177-179; and U.S. patent application Ser. No. 556,305, "Birefringent Fiber Narrow Band Polarization Coupler", filed Nov. 30, 1983, and assigned to the assignee of the present application (now abandoned). Each of the foregoing articles and the patent application are incorporated herein by reference.
It has also been found that if an optical fiber having two paths for propagation of an optical signal is periodically stressed by a traveling surface wave (e.g., a traveling surface acoustic wave) or a simulated traveling wave, the light energy of the optical signal traveling along one propagation mode can be coupled to another propagation mode and shifted in frequency by the magnitude of the frequency of the traveling wave. The frequency-shifted optical signal can be separated from the unshifted optical signal by using a modal coupler or a polarizer. Such frequency shifting devices have been described in the following articles: "Acousto-Optic Frequency Shifter for Single Mode Fibers," Nosu, et al., published at 47th International Conference on Integrated Optics and Optical Fiber Communications in Tokyo, June 27-30, 1983, and in ELECTRONICS LETTERS, Vol. 19, No. 20 (29 September, 1983); "Single-Sideband Frequency Shifting in Birefringent Optical Fiber", W. P. Risk, et al., SPIE, Vol. 478, FIBER OPTICS AND LASER SENSORS II, May, 1984, pp. 91-97; "Acousto-Optic Frequency Shifting in Birefringent Fiber," W. P. Risk, et al., OPTICS LETTERS, Vol. 9, No. 7, July 1984, pp. 309-311; "Acousto-Optic Birefringent Fiber Frequency Shifters," W. P. Risk, et al., INTEGRATED AND GUIDED WAVE OPTICS CONFERENCE, sponsored by the Quantum Electronics Group of IEEE and by the Optical Society of America, Kissimmee, Fla., (Apr. 24-26, 1984); and "Acoustic Fiber-Optic Modulators," W. P. Risk, et al., PROCEEDINGS OF THE IEEE ULTRASONICS SYMPOSIUM, Nov. 14-16, 1984, pp. 318-327. Acousto-optic frequency shifters are also disclosed in copending U.S. patent application Ser. No. 556,636, "Single Mode Fiber.Optic Single Sideband Modulator," filed Nov. 30, 1983 (U.S. Pat. No. 4,684,215); in copending U.S. patent application Ser. No. 581,176, "Acousto-Optic Frequency Shifter", filed on Feb. 17, 1984 (U.S. Pat. No. 4,735,485); and in "Acousto-optic Frequency Shifter Utilizing Multi-Turn Optical Fiber," U.S. patent application Ser. No. 699,666, filed on Feb. 8, 1985, (U.S. Pat. No. 4,735,484) all of which are assigned to the assignee of the instant application. The foregoing articles and patent applications are incorporated herein by reference.
As discussed in copending patent application Ser. No. 556,636 (U.S. Pat. No. 4,684,215), one approach to frequency shifting is to launch an actual acoustic wave (either a surface wave or a bulk wave) for propagation longitudinally along the length of an optical fiber. This approach has the advantage of providing a continuous, virtually infinite, number of coupling points which travel along the length of the fiber. However, it has been found that in order to achieve maximum coupling between propagation modes in an acousto-optic frequency shifter constructed in accordance with U.S. patent application Ser. No. 556,636 (U.S. Pat. No. 4,684,215), the acoustic wavelength should be substantially equal to the beat length of the optical energy traveling in the two propagation modes of the optical fiber. For example, in an exemplary commercially available high birefringence fiber, the minimum beat length between the optical energy traveling in the two polarization modes of the fiber is on the order of 1 mm. An acoustic wavelength of 1 mm corresponds to an acoustic frequency of about 1-5 MHz, depending upon the acoustic propagation velocity in the acoustic propagation medium (e.g., 3.4 MHz at 3411 meters/sec.). In order to obtain greater shifts in the optical frequency, a higher acoustic frequency must be used. However, since a higher acoustic frequency corresponds to a shorter acoustic wavelength, phase matching between the acoustic wave and the beat length of the optical fiber cannot be obtained by directing the acoustic wave colinearly with the propagation direction of the optical signals in the optical fiber. Thus, a number of techniques have developed in the art for propagating the acoustic wave at an angle with respect to the direction of propagation of the optical signal in the optical fiber. For example, the above-referenced copending patent application Ser. No. 581,176 (U.S. Pat. No. 4,735,485) discloses a fiber optic frequency shifter in which an acoustic transducer is positioned at an angle relative to an optical fiber such that the acoustic wave fronts of the acoustic wave generated by the transducer acoustically contact the fiber at an angle of incidence which is less than 90.degree. and greater than 0.degree.. When the acoustic wave fronts of the acoustic wave acoustically contact the fiber at an angle of incidence of 90.degree., the acoustic wave is propagating colinearly with the optical signals (i.e., the acoustic wave is propagating in the same direction as the optical signals) and when the acoustic wave fronts acoustically contact the fiber at an angle of incidence of 0.degree., the acoustic wave is propagating in a direction perpendicular to the direction of propagation of the optical signals. The angle of incidence can be chosen such that the shorter wavelengths of higher frequency acoustic waves can be matched with the longer beat lengths of the optical signal in the fiber. Thus, the maximum possible frequency shift can be substantially increased. A number of techniques are disclosed in U.S. Pat. application Ser. No. 581,176 (U.S. Pat. No. 4,735,485) to increase the concentration of acoustic energy impinging upon the optical fiber. For example, U.S. patent application Ser. No. 699,666 (U.S. Pat. No. 4,735,484) discloses an alternative apparatus for increasing the percentage of acoustic energy applied to the optical fiber by wrapping the optical fiber around the acoustic propagation means so that the propagating acoustic waves contact the optical fiber at a plurality of locations. The two techniques described in the two copending applications are further described in "Acousto-Optic Birefringent Fiber Frequency Shifters," W. P. Risk, et al., INTEGRATED AND GUIDED WAVE OPTICS CONFERENCE, sponsored by the Quantum Electronics Group of IEEE and by the Optical Society of America, Kissimmee, Fla., (Apr. 24-26, 1984); "Single-Sideband Frequency Shifting in Birefringent Optical Fiber", W. P. Risk, et al., SPIE, Vol 478, FIBER OPTICS AND LASER SENSORS II, May, 1984, pp. 91-97; "Acousto-Optic Frequency Shifting in Birefringent Fiber," W. P. Risk, et al., OPTICS LETTERS, Vol. 9, No. 7, July 1984, pp. 309-311; and "Acoustic Fiber-Optic Modulators," W. P. Risk, et al., PROCEEDINGS OF THE IEEE ULTRASONICS SYMPOSIUM, Nov. 14-16, 1984, pp. 318-327.
The devices described in the foregoing articles and copending patent applications require alignment of the optical fiber at an angle with the direction of propagation of the acoustic wave in order to achieve proper phase matching. In order to achieve the angular alignment, the acoustic wave must have broad wave fronts in order to affect a substantial length of the optical fiber. For an acoustic wave of a given width, the closer that the direction of propagation of the acoustic wave is to being perpendicular to the direction of propagation of the optical signal, the more power is required to couple optical energy from one propagation mode to the other propagation mode. Thus, a large amount of acoustic energy must be used to obtain substantial acoustic coupling. Alternatively, as described in copending patent application Ser. No. 699,666 (U.S. Pat. No. 4,735,484), the optical fiber can be precisely wound around the acoustic propagation medium so that the propagating acoustic wave provides a cumulative effect each time it passes across the optical fiber at the selected angle.